left Rear speaker vibrates

[HCguy]

I've had this problem for a while but only decided to take care of it this week. the rear left speaker or the panel of that door is vibrating with certain bass. This speaker is the only one to vibrate of all 4. I went to the Regan dealer here in Long Island City, NY and they told me they checked the panel and the speaker's screws and couldn't replicate it ( I tried and tried my best to do it for them but couldn't replicate it either) the next day it started up again. Anybody have an idea? I'm thinking it's something in the speaker itself that's loose.

[HCguy]

Thanks for the help. Weird that it didn't want to vibrate when I needed it to lol.